Thanks.

I don't think Darcy had told the Col. about his proposal, and I can't think of another reason why he would reveal the truth about his sister. I think his pride would certainly have kept him from revealing that he had been refused, if the Col. were not already waiting to congratulate him.

My idea is that Col. F. was very well-mannered and felt it was due their friendship to say good-bye to Lizzy if he could. Also, I think he liked her. This would be a farewell to the wishes he might have had that he were not a younger son, before he went off and looked for a young lady of fortune to court (not that I think he would act as crudely as I make it sound). He may never be able to speak with her again as openly and intimately as he did at Rosings.